Switch to using allowed_mimetypes in webapi resources.

The allowed_item_mimetypes and allowed_list_mimetypes were deprecated in
Djblets 0.6, but a few of our resources still use them. They'll be nuked
from Djblets soon, so I've updated our code here to no longer use them.
Remove positional and unnecessary arguments on get_queryset calls.

A number of API handlers were passing positional arguments to
get_queryset, such as the review_request_id. Most of these arguments
weren't even necessary to explicitly pull out of kwargs and pass, since
we're passing kwargs directly.

The problem with positional arguments in these calls is that things get
messed up if a get_queryset or get_object implementation wants to
declare some other parameter before args and kwargs. The wrong value
would get passed for the wrong argument.
Rewrite the documentation for selecting diff revisions.

Our nice new revision selector UI is hopefully pretty self explanatory, but all
the same, we should document it. The nice thing about this new UI is that it's
much easier to explain how to choose an interdiff.

Testing done: Generated HTML docs.
Reviewed at http://reviews.reviewboard.org/r/4783/